Duke called Elsa on the phone and told her that he needed 20 bales of hay (about $300 worth). Elsa told Duke that she had the hay, and would deliver it the next day, and true to her work, the next day Elsa delivered 20 bales of hay to Duke's, but as she was unloading the last bale, Duke said, "I don't need the hay- take it back." Valid contract? Is Duke obligated or not? Explain fully